Environmental concerns have led to decreases in the permissible levels of sulfur in hydrocarbon fuels. Sulfur in refinery streams, e.g., feedstocks, is present in a number of different forms, including aliphatic and aromatic compounds. Sulfur, however, tends to be concentrated in the higher boiling fractions, mainly in the form of aromatic heterocycle compounds such as benzothiophenes, and dibenzothiophenes
Refiners have employed catalytic hydrodesulfurization processes to reduce sulfur in hydrocarbon fuel feedstock. Conventional hydrodesulfurization processes are capable of removing sulfur compounds, especially the lower molecular weight materials including mercaptan sulfur-containing aliphatic materials and thiophenes to levels of <30 ppm. Hydrodesulfurization processes can also reduce the more refractory sulfur compounds, but only at higher desulfurization severities, increased cost and with greater difficulty.
Petroleum refiners are now in the process of devising methods for making ultra low sulfur gasoline in order to comply with increasingly stringent environmental regulations. In certain countries, regulations require refiners to produce gasoline containing 50-ppm sulfur or less by 2005, and in some countries refiners may have to produce gasoline containing less that 10-ppm sulfur beginning in 2008. Some countries have already introduced tax-incentives for 10-ppm gasoline sulfur levels. Regulations requiring these ultra low sulfur levels will incur great expense in terms of capital expenditures and increased refinery operating costs if the refiner relies on current hydrodesulfurization technology.
Refiners are considering several factors when designing new processes and facilities for meeting these new regulations. Those factors include the required level of sulfur reduction, existing refinery equipment that might be retrofitted/used, overall cost, operational flexibility, simplicity of reconstructing the plant for possible lower sulfur specifications in the future and commercial operating experience of the technology to be used.
Many refiners, particularly those with FCC units producing gasoline having high sulfur levels, have already made strategy and investment decisions for 50-ppm sulfur gasoline. One process being considered by refiners with FCC units is CDTECH, Inc.'s two-zone unit comprising a CDHydro unit and a CDHDS unit. In particular, CDHydro and CDHDS are used to selectively reduce sulfur in naphtha feedstock leaving a fluidized catalyst cracking (FCC) unit with minimum octane loss which is typically seen when employing other catalytic hydrodesulfurization processes to reduce sulfur content. The CDTECH process treats light, mid and heavy cat naphthas (LCN, MCN, HCN), with each fraction treated under optimal sulfur reduction conditions.
The overall CDTECH process begins in the CDHydro unit wherein the FCC naphtha is subject to catalytic distillation. The CDHydro is designed to fractionate the naptha into a low sulfur, low boiling point fraction and a higher sulfur content, higher boiling point fraction. More specifically, catalyst is provided in the CDHydro unit to catalyze the reaction of sulfur-containing aliphatics, e.g., mercaptans, with excess diolefins to produce heavier thioether compounds that will not fractionate with the lighter boiling olefin overhead. The remaining diolefins are partially saturated to olefins by reaction with hydrogen, which is also present in the first zone. The conditions of the CDHydro unit are set at endpoints of about 70° C. so that higher boiling point sulfur species, such as thiophenes and benzothiophenes, in the FCC naptha will not fractionate as part of the lighter boiling point overhead. These species, along with the thioethers, will be present in the CDHydro high boiling bottoms product.
Bottoms from the CDHydro column are then fed to a second zone, i.e., a CDHDS column, where the bottoms are catalytically desulfurized in the presence of hydrogen. The hydrodesulfurization conditions are optimized to achieve the desired sulfur reduction with minimal olefin saturation. Olefins are concentrated at the top of the CDHDS column, where conditions are mild, while sulfur is concentrated at the bottom where the conditions result in very high levels of hydrodesulfurization.
The product streams from the two zones are stabilized together or separately, as desired, resulting in product streams appropriate for their subsequent use.
While the CDTECH process has shown to effectively reduce sulfur in naphtha feeds, they do require significant capital investment and relatively high operating costs, with a significant portion of these costs relating to the CDHDS unit and its operation. Furthermore, only 40% of FCC gasoline is passed into CDHydro's overhead, thereby subjecting a significant portion of the FCC gasoline's olefin content to hydrogen and saturation in the CDHDS zone. Accordingly, refiners selecting a CDTECH process to meet the new sulfur regulations are facing significant expenses.
Membrane processes have also been suggested for reducing sulfur content in hydrocarbon feedstocks. Published Patent Application 2002/0153284, published on Oct. 24, 2002, describes employing membranes to reduce sulfur content of a naphtha feedstream from a FCC unit. A membrane is selected so that when the sulfur-containing naphtha is contacted with the membrane, a sulfur rich permeate is created on one side of the membrane while a sulfur deficient retentate is created on the other side of the membrane. The retentate is then processed further as a low sulfur product stream, while the permeate is routed to a traditional sulfur reduction unit. Membrane units, usually in the form of modules, are relatively inexpensive, and are an excellent choice for those refiners who have not yet invested capital in another type of sulfur reduction. Currently available membranes, however, do not remove certain sulfur species, e.g., mercaptans, as effectively as aromatic sulfur species. There may be a benefit of employing other technologies with membranes when faced with removing significant amounts of mercaptans.
For those refiners who have already selected a capital intensive sulfur reduction process, it would be highly desirable to find a way to defray and/or reduce the costs of the process, either by lowering costs to operate the equipment, reducing the wear and tear on the same or reducing the cost of replacement equipment when a piece of equipment has failed. Additionally, it is expected that the requirement to debottleneck the capital intensive sulfur reduction process will create opportunities for new less expensive processes used in concert with the originally selected technologies.